Using X-ray photoelectron spectroscopy, the oxidation/alteration of massive coexisting pentlandite (Pn) and pyrrhotite (Po) from Voisey’s Bay has been studied in aqueous solution at pH 9.3, with the addition of the xanthate collector potassium amyl xanthate (KAX) at varying solution O2 concentrations. As in the accompanying XPS study, the oxidation/alteration of the two mineral surfaces is monitored by the Fe 2p, Ni 2p, S 2p, and O 1s XPS spectra. The major surface product on all surfaces is FeOOH. However, the amount of FeOOH depends greatly on the presence of KAX, the presence of ground Pn/Po of high surface area, and the O2 solution concentration. At air-saturated O2 solution concentrations of 8–9 ppm, the amount of FeOOH on both surfaces decreases in the presence of KAX compared to the situation without KAX. In the presence of the Pn/Po powder, the oxidation of Pn decreases still more, suggesting that the powder scavenges O2, and that the oxidation of Pn is more sensitive to O2 concentration than the oxidation of Po. When the O2 concentration is decreased drastically to <0.01 ppm (by Ar purging) in the presence of KAX, both minerals are relatively free of FeOOH: indeed, both minerals show less FeOOH than the unreacted sample, which implies that the mineral surfaces are being “cleaned” under these conditions. The degree of oxidation of both surfaces increases with increase in the O2 solution concentration; but at about 0.03 ppm O2 the pyrrhotite surface is heavily oxidized, while the pentlandite surface remains largely unoxidized, and less oxidized than the initial unreacted sample. This result strongly indicates that the flotation separation of Pn and Po could be enhanced at low O2 concentrations.